The invention relates to a bushing structure having a polygonal flange. An example of such a bushing structure having a polygonal flange is disclosed in the U.S. Pat. Nos. 1,513,638 and 1,982,145, respectively. Usually such a bushing structure is provided with an internal thread for a screw plug and is used in the wall of a barrel in which a polygonal impression has been made for receiving the polygonal flange. The flange and the impression cooperate in order to keep the bushing in a fixed position when screwing a screw plug in its place in the bushing.
When producing such a bushing structure having a flange the flange portion is made by starting from a blank punched out of sheet metal, whereby in view of technical reasons when machining it is necessary that the dimensions of the blank are somewhat bigger than the final dimensions of the flange portion. Successive machining operations are utilized to form the bushing and the flange portion and to impart the correct dimensions whereby the polygonal flange is obtained by subsequently punching away the edge portions.
The above described way of manufacture leads to material losses. This invention envisages a bushing wherein material losses are avoided.
Punching operations entail the drawback that sharp edges may be formed which detrimentally affect the adhesion of a coating if the bushing is provided with a coating. Sharp edges may be avoided by frequent and accurate maintenance of the punch dies and/or additional operations. This invention furthermore envisages a bushing in which the development of sharp edges, particularly at the periphery of the polygonal flange portion is completely eliminated in a more simple manner.
According to the invention this object is attained by initially forming the flange of the bushing structure with an outer diameter which is circular and substantially equal to the diameter of the inscribed circle of the polygon to be made, then subsequently cold working the material of the flange at the location of the angles of the polygon to be made. This working is by means of a tool acting transversely on the plane of the flange in order to impart a forced outward flow to the material to an extent that the flow is the largest at the location of the angles and decreases in the direction of the points lying between said angles. Hence the invention is based on the conception of starting from a circular flange of too small a size followed by imparting thereto a polygonal shape by local impression.
Otherwise than upon punching this operation is not conducive to sharp edges. Moreover the material is strengthened by the cold working operation.
The bushing portion running from the flange is made by removing a circular center portion from the blank punched out of sheet metal and converting the inner edges into a cylindrical sleeve. At a later step the cylindrical sleeve is provided with an internal screw thread. These are generally known operations; it is novel however that out of the sheet metal there is punched a blank which is smaller than the blank employed by known methods but the final product, after subsequent processing, is just as large as that made by present methods.